MacBook – Time machine deletes old backups when still >2 TB of 3 TB are free

The system will delete old backups if it has an estimate that it will need the space for the impending backup. Normally the system over estimates a backup by 2 to 4 GB in routine use, but I have seen some cases where it had estimates of 250 GB or more due to some folder changes. Even without an error in estimation, you could have attached an external drive that increased this estimate drastically.

Only you have the logs and data to know how plausible this is. Check out BackupLoupe if you like to have better measurements on how each backup differs in size from the others.

For this reason, I periodically duplicate my Time Machine drives (or choose to rotate them monthly) so that if there is a bug, I can look when I swap in the new drive to make sure I don't lose some of my older history.

Time Machine is using differential backups. That means your first backup is a complete backup which is same size with your drive. Starting from the second one backup will include changed files since last backup.

These Backups are accessible by finder. So you can easily find out which folders causes these big backups. After you find the folders you can manually exclude these folders from Time Machine Backup Options.

Some softwares may cause huge backups if they changes files last updated status like Dropbox Agent or similar. You may consider exclude folders of these softwares because they are already copied to the cloud.